The invention relates to a process for making fibres from a polyamide consisting wholly or for the most part of poly(p-phenylene terephthalamide) in which a spinning mass is prepared by cooling concentrated sulphuric acid of at least 98 wt. % content to below its coagulation point, subsequently combining the thus cooled sulphuric acid with the polyamide and mixing them to form a solid mixture containing, calculated on the weight of the mixture, at least 15% of the polyamide having an inherent viscosity of at least 2.5, and then heating the resulting solid mixture and passing it, under pressure, to spinning orifices, and spinning it by means of an air gap spinning process.
Such a process is known from Netherlands patent application 7904495 laid open to public inspection. It was found that with the aid of this so-called freezing process proper mixing of sulphuric acid and poly(p-phenylene terephthalamide) could be achieved. However, the resulting sandy solution lacks sufficient homogeneity for immediate processing after being melted. For this reason the sandy solution is heated in an extruder prior to being spun. Not all drawbacks are obviated by this procedure, however. Since not every polymer granule will absorb the same quantity of sulphuric acid, the sulphuric acid:polymer ratio on a microscopic scale differs from the set macroscopic scale ratio. This will give inhomogeneities during melting. Furthermore, it was found that favourable results in actual practice are highly dependent on precise settings for the pressure build-up and the variation in temperature in the extruder. In consequence, there is a considerable risk of failure of the extruder process. It was also found to be advisable in actual practice to compress the sandy solution in the cold state, which severely curtails the options to increase or change the process capacity.
It has also been proposed to prevent inhomogeneities in the poly(p-phenylene terephthalamide) spinning solution by means of an improved process of dissolving poly(p-phenylene terephthalamide) in sulphuric acid. A two-step mixing process to this end in which poly(p-phenylene terephthalamide) and sulphuric acid are successively mixed in a small twin-screw mixer operated at high speed and a large twin-screw mixer operated at low speed is known from Research Disclosure 232 004. Such a process has nothing to do with eliminating the drawbacks to the known freezing process, nor does it have the advantages provided by said process.
From WO 92/07120 it is known to modify a poly(p-phenylene terephthalamide) spinning solution by the addition thereto of solution containing additives. A static mixer unit is employed to this end. This process has nothing to do with improving the aforementioned freezing process either.